Touch the All Apps icon on your Home screen (white circle with 6 squares)
Touch Widgets at the top of the screen.
From the main Widgets screen, you can swipe left until you find "Countdown Widget"
Touch and hold the widget's icon, slide your finger where you want to place it, and lift your finger. Widget configuration dialog will appear.
Note: If there isn't room where you dropped it, the widget won't get added to the Home screen.